1. Field of the Invention
The present invention relates to predictive coding for a video encoding system, and more particularly, to a video predictive coding apparatus and method thereof, which can enhance the coding efficiency by predictively coding DC coefficients of a block to be coded using DC gradients of a plurality of previously coded neighboring blocks.
2. Description of the Related Art
In general, in conventional standards for video data compression such as MPEG-1, MPEG-2, JPEG, H.261, H.263 and so on, which adopts a block-oriented coding method, intra-mode blocks (to be referred to as intra blocks, hereinafter) which does not use a time- directional prediction, are coded and transmitted to a decoder by a texture coding using a 8H8 DCT (Discrete Cosine Transform) for reducing spatial redundancy of a signal.
Considerable parts of overall data to be transmitted are generated during the texture coding. Thus, the performance of an encoder is greatly affected by efficiently coding the data generated during the texture coding.
According to the H.263 method, in the case of an intra block, DC coefficients are quantized with a quantization step size of 8 into 8 bits after performing an 8H8 DCT, transmitted by performing a PCM (Pulse Code Modulation) on the same. According to the MPEG-2 method, to enhance the coding efficiency of DC coefficients, a difference between a quantized DC coefficient of a directly previous block having a relatively high spatial correlation and that of a current block, is transmitted in zig-zag scanning (luminance blocks) and progressive scanning (chrominance block) orders, as shown in FIG. 2.
Here, the zig-zag scanning and progressive scanning are dependent upon the coding order of a macroblock (16H16 pixels) in a frame.
In the MPEG-2, if the X-axis coordinate of the macroblock to be coded is 0, that is to say, if there is no block previously coded spatially), the predictive coded value (DC value), for the first luminance block in the macroblock, is 128, as shown in FIG. 2. Also, for the chrominance block, the DC value is 128.
The 8H8 DCT is performed in an encoder, as expressed in the following equation (1), and IDCT (Inverse DCT) is performed in a decoder (and decoding parts for the reconstructed signal in the encoder), as expressed in the following equation (2). After performing DCT, DC and AC coefficients are arranged, as shown in FIG. 1.
Here, the DC coefficients correspond to F(0,0) of the equation (1), and the AC coefficients correspond to the remaining ones except the F(0,0). ##EQU1##
where u, v, x and y are 0, 1, 2, . . . 7, respectively, x and y are spatial positions in a pixel domain, u and v are spatial positions in a transform domain, u and x are horizontal indices, and v and y are vertical indices. Also, C(u),C(v)=1/2+L for u,v=0, and 1 otherwise (See ITU-T Recommendation H.263 Annex A: Inverse Transform Accuracy Specification, 6.2.4: Inverse Transform).
Also, f(x, y) represents video signal values within a 8H8 block, and F(u, v) are DC and AC coefficients calculated from the equation (1). Also, F(u, v) of the equation (2) are DC and AC coefficients coded by the coder.
FIG. 3 shows a macroblock with a 4:2: Format, in a block-oriented coding method used in the conventional standards such as MPEG-1, MPEG-2, H.263, etc.
Here, the 4:2:0 video format represents ratio of sampling frequencies of three components, luminance information Y, and chrominance information Cb and Cr. The ratio of 4:2:0 is taken as a representative value of 4:2:0 and 4:0:2, which are alternative ratios of odd and even lines.
Also, the macroblock is composed of each 16 pixels horizontally and vertically, and is largely classified into four luminance blocks, i.e., L1 (Block 1 of luminance), L2 (Block 2 of luminance), L3 (Block 3 of luminance), and L4 (Block 4 of luminance), and two chrominance blocks, i.e., Cb and Cr. The respective pixels of C1(=Cb) and C2(=Cr) correspond to the positions of luminance components subsampled horizontally and vertically at the ratio of 2:1, which is referred to as 4:2:0 video format, as shown in FIG. 3.
In the MPEG-2 or MPEG Video verification model version 2, during a texture coding, in the case of an intra frame (or intra picture) having a high spatial (intra-picture) correlativity of luminance and chromance values, DC predictive coding is performed in the consideration of only one neighboring block Because this prediction method does not sufficiently consider the characteristics of texture signals of the neighboring blocks of the block to be coded, but only the coding order of the block or the macroblock as mentioned above, the coding efficiency of the texture coding (especially DC coefficients) is low.